# std::expected<T,E>::transform

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< cpp‎ | utility‎ | expected

Primary template |  |
template< class F > constexpr auto transform( F&& f ) &; | (1) | (since C++23)
template< class F > constexpr auto transform( F&& f ) const&; | (2) | (since C++23)
template< class F > constexpr auto transform( F&& f ) &&; | (3) | (since C++23)
template< class F > constexpr auto transform( F&& f ) const&&; | (4) | (since C++23)
void partial specialization |  |
template< class F > constexpr auto transform( F&& f ) &; | (5) | (since C++23)
template< class F > constexpr auto transform( F&& f ) const&; | (6) | (since C++23)
template< class F > constexpr auto transform( F&& f ) &&; | (7) | (since C++23)
template< class F > constexpr auto transform( F&& f ) const&&; | (8) | (since C++23)

If *this represents an expected value, invokes f and returns a `std::expected` object that contains an expected value, which is initialized with its result (or value-initialized if the result type is void). Otherwise, returns a `std::expected` object that contains an unexpected value, which is initialized with the unexpected value of *this.

1-4) f is invoked with the expected value `val` of *this as the argument.
5-8) f is invoked without any argument.

Given type `U` as:

1,2) std::remove_cv_t<std::invoke_result_t<F, decltype((`val`))>>
3,4) std::remove_cv_t<std::invoke_result_t<F, decltype(std::move(`val`))>>
5-8) std::remove_cv_t<std::invoke_result_t<F>>

If any of the following conditions is satisfied, the program is ill-formed:

- `U` is not a valid value type for `std::expected`.
- std::is_void_v<U> is false, and the following corresponding declaration is ill-formed:

1,2) U u(std::invoke(std::forward<F>(f),`` `val`));
3,4) U u(std::invoke(std::forward<F>(f), std::move(`val`)));
5-8) U u(std::invoke(std::forward<F>(f)));

1,2) These overloads participate in overload resolution only if std::is_constructible_v<E, decltype(error())> is true.
3,4) These overloads participate in overload resolution only if std::is_constructible_v<E, decltype(std::move(error()))> is true.
5,6) These overloads participate in overload resolution only if std::is_constructible_v<E, decltype(error())> is true.
7,8) These overloads participate in overload resolution only if std::is_constructible_v<E, decltype(std::move(error()))> is true.

### Parameters

f | - | a suitable function or Callable object whose call signature returns a non-reference type

### Return value

Given expression expr as:

1,2) std::invoke(std::forward<F>(f),`` `val`)
3,4) std::invoke(std::forward<F>(f),std::move(`val`))
5-8) std::invoke(std::forward<F>(f))

The return values are defined as follows:

Overload | Value of `has_value()`
--- | ---
true | false
(1,2) | If std::is_void_v<U> is false, returns std::expected<U, E> `` (std::in_place, expr). Otherwise, returns std::expected<U, E>() (but still evaluates expr). | std::expected<U, E>(std::unexpect, error())
(3,4) | std::expected<U, E> `` (std::unexpect, std::move(error()))
(5,6) | std::expected<U, E>(std::unexpect, error())
(7,8) | std::expected<U, E> `` (std::unexpect, std::move(error()))

### Example

| This section is incomplete Reason: no example

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3938 | C++23 | the expected value was obtained by value()[1] | changed to **this
LWG 3973 | C++23 | the expected value was obtained by **this[2] | changed to `val`

1. ↑ `value()` requires `E` to be copy constructible (see LWG issue 3843), where `operator*` does not.
2. ↑ **this can trigger argument-dependent lookup.

### See also

transform_error | returns the `expected` itself if it contains an expected value; otherwise, returns an `expected` containing the transformed unexpected value (public member function)

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